In order to produce a better ride on a vehicle, the damping force of the shock absorbers on the vehicle should be varied, depending on the road conditions and on the operating conditions of the vehicle. An automotive shock absorber equipped with such a damping force control mechanism is described in Japanese Patent Laid-Open No. 194609/1983, for example. In this known shock absorber, a piston is fitted in a cylinder to partition the inside of the cylinder into two hydraulic chambers. A passage is formed to place the hydraulic chambers in communication with each other. The damping force of the absorber is varied by changing the area of the passage by a rotary valve. In this prior art method for adjusting the damping force of the absorber according to the road conditions and the operating conditions of the vehicle, the conditions of the road surface and the operating conditions of the vehicle are estimated, using various sensors installed at various locations in the vehicle to control the damping force. That is, the road conditions and the operating conditions of the vehicle are known indirectly.
In the prior art method, the conditions of the road surface that the vehicle is meeting and the operating conditions of the vehicle are detected by the numerous sensors and, therefore, a very expensive control apparatus is necessary for the shock absorber in order to produce a variable damping force. Also, a sensor which detects the acceleration of the shock absorber does not exist. Consequently, the acceleration of the shock absorber itself is not taken into account in controlling the communication between the two hydraulic chambers formed in the cylinder.